Generally, a gimbal is used for mounting and fixing an image pick-up device such as a video camera, a camera and so on, a motion of the gimbal is controlled by an electric motor, and then a position of the image pick-up device mounted on the gimbal is adjusted, to achieve scanning, image pick-up and monitoring in a wide range. In order to protect the image pick-up device, a transparent cover is provided thereon, to prevent the image pick-up device from being damaged during use. However, since ambient light is refracted after passing through the transparent cover, when the image pick-up device shoots in different positions in the transparent cover, captured image has defects such as distortion, blurring, uneven brightness and so on under the influence of light.